The invention relates to a thermal regulating composition that can be used, for example, as a catalyst system in a fuel processor for a fuel cell system.
Fuel cells are an increasingly popular power generation technology, where chemical reactions are utilized to produce electricity. The reactants are typically hydrogen and oxygen. Along with the electricity generated, the sole reaction product is water. Hydrogen for such fuel cells may be supplied by chemically converting a fuel such as natural gas, propane, gasoline, diesel, methanol, etc., into a hydrogen-rich stream. This process is typically referred to as fuel processing, and the hydrogen-rich stream is typically referred to as reformate.
The catalyst systems used in fuel processors generally include a dispersion of small catalyst particles on a support material. It is generally desirable to minimize the size of the catalyst particles that are used in order to maximize the surface area of catalyst that is provided to promote a given reaction. However, small metal particles, such as those typically used as catalysts, may tend to be pyrophoric, meaning that they will spontaneously and rapidly oxidize when exposed to oxygen or air. Oxidation is exothermic, meaning that the reaction releases heat energy. Pyrophoricity tends to increase as smaller particles are used, and some metals (e.g., non-precious metals such as iron and copper) may tend to be more pyrophoric than others. In some cases, the heat generated by this oxidation may pose a fire or other safety hazard, or may damage the catalyst configuration itself.
Catalysts subject to such concerns are typically pre-reacted with oxygen in a controlled environment before they are handled. For example, a catalyst may be oxidized slowly in a dilute oxygen atmosphere to avoid overheating, and may then be shipped in a relatively non-reactive oxidized state (referred to as the oxidized state). Since catalysts in an oxidized state generally have diminished catalytic effectiveness or no effectiveness at all, they are typically reduced or activated before they can be used (referred to as the reduced, or active state). This generally involves flowing hydrogen or another reducing agent across the catalyst at an elevated temperature (e.g., over 200xc2x0 C.), in order to react away the oxidation layer. This reduction (activation) step is also exothermic, and may need to be controlled (e.g., by using diluted hydrogen) to avoid overheating.
One reason why catalyst overheating can be a problem, and thus why catalyst temperature control is important, is because some catalysts will lose their catalytic effectiveness if they are overheated. For example, when copper-based catalyst particles are heated to over 400xc2x0 C., the particles may tend to sinter (also referred to as densification), meaning that small particles will tend to combine into larger particles. Thus, this temperature may be referred to as the sintering temperature of this material. Such sintering can reduce the surface area of the catalyst, thereby reducing its effectiveness. As known in the art, other catalyst materials are subject to similar concerns at other sintering temperatures.
The invention relates to a thermal regulating composition that can be used, for example, as a catalyst system in a fuel processor for a fuel cell system.
In general, in one aspect, the invention provides a composition including a first material capable of catalyzing or undergoing an exothermic chemical reaction, and a second material capable of sorbing and desorbing a heat transfer material. The second material is present in an amount sufficient to sorb an amount of the heat transfer material sufficient to remove heat from the first material when heat from the exothermic reaction causes the heat transfer material to desorb from the second material. The first material and the second material may form a mixture.
In certain embodiments, the second material can be a desiccant, such as a zeolite, silicon oxide, aluminum oxide, or a clay. In such embodiments, the heat transfer material is water. The first material can be a fuel processor catalyst, such as a material or compound including copper, nickel, iron, chromium, zinc, cobalt, platinum, palladium, rhodium, ruthenium, or iridium. Fuel processor catalysts can be classified, for example, as catalytic partial oxidation catalysts, high temperature water-gas shift catalysts (also generally referred to as the shift reaction), low temperature water-gas shift catalysts (also generally referred to as the shift reaction), and preferential oxidation catalysts.
An advantage is that this configuration reduces the pyrophoricity of the composition such that in some embodiments the composition can be exposed to air without hazard to safety or damage to the catalyst configuration. The finely-divided catalyst material, consequently, can have a high surface area and oxidize, for example, when air accidentally enters into the reformer, without damage to the catalyst. Thus, in some embodiments, the invention provides a catalyst material that is easier and safer to handle, and can be oxidized (as for shipping) or activated more easily, safely and conveniently than catalyst systems not of the invention. Another advantage is that this temperature quenching capability of the composition allows superior temperature control of the composition during reaction. For example, compositions under the invention are less prone to rapid temperature excursions from runaway reactions than compositions not of the invention. The temperature of compositions under the invention have improved controllability through heat exchange methods and reactant control (such as cutting off or reducing reactants), as examples.
In general, some embodiments of the invention provide a catalyst that includes a sufficient amount of a desiccant to sorb and desorb water to substantially quench the temperature of the catalyst when the catalyst is exposed to an exothermic reaction. Under normal operating conditions, the desiccant sorbs water from the reformate stream. However, when the catalyst is exposed to an exothermic oxidizing or reducing condition, the heat generated causes the water in the desiccant to desorb as vapor into the reformate, thereby cooling the catalyst and making the system more temperature controllable. The desorbed, vaporized water also permeates throughout the catalyst tending to further cool the catalyst. As an example, some desiccants, such as zeolites, can desorb up to about 50-100 L of steam per one liter of desiccant. Thus, the risk of the catalyst overheating and/or igniting is minimized with the desorption/evaporative cooling capacity of this water, thereby lessening the risk of damage to the catalyst material and other components in the reformer.
The water sorption by the desiccant may also temporarily enhance the performance of the catalyst by allowing the desiccant to supply more water to the catalyst material. More water generally enhances the water-gas shift reaction by shifting the reaction to an equilibrium favoring the production of hydrogen and carbon dioxide, thereby producing more hydrogen while reducing CO.
In certain embodiments, the first material (generally the catalyst) and the second material (generally the desiccant) are present in a range of weight ratios from about 1:1 to about 1:10, respectively. In other embodiments, a narrower range may be desired, such as from about 1:1 to about 1:5. The first and second materials may also be present in approximately equal amounts. In yet other embodiments, it may be desired to have more of the first material than second material, or to have even more of the second material present than described above, such as a ratio of over 1:10.
In certain embodiments, the first and second materials are formed into granules. The granules may have spherical, cylindrical, or other shapes. The granules can also be pellets and agglomerated particles. The first and second materials may also be coated onto a support structure, such as a ceramic monolith. In some embodiments, the first material may be coated onto the second material. In other embodiments, the first material may be disposed on a third material. The third material may include, as examples, an aluminum oxide, zinc oxide, zirconium oxide, or an iron oxide. As examples, it may be desired to have the first material disposed on the third material with a load between about 5% and about 30% by weight, or a load between about 10% and about 20% by weight. It may also be desired for the first and third materials to form a catalytic composite containing less than about 20% by weight of the third material, such as about 5% to about 15% by weight of the third material.
In still other embodiments, it may be desired for the catalytic composite to contain a fourth material such as zinc oxide, zirconium oxide, and iron oxide. It may be desired for the catalytic composite comprises less than about 50% by weight of the fourth material, such as between about 20% and about 30% by weight of the fourth material. The composition can also include a fifth material capable of sorbing hydrogen and forming hydrogen atoms, such as platinum, nickel, palladium, rhodium, or ruthenium.
In general, in another aspect, the first material can have a reduced state and an oxidized state, and the second material can be present in an amount sufficient to prevent the composition from sintering when the first material is in the reduced state and is exposed to air. The second material can also be present in an amount sufficient to prevent the composition from reaching a sintering temperature of the first material when the first material has a starting temperature of less than 100xc2x0 C. and is in the reduced state and is exposed to a flow of 4 liters per minute of air. The second material can also be present in an amount sufficient to prevent the composition from reaching a sintering temperature of the first material when the first material has a starting temperature of less than 100xc2x0 C. and is in the oxidized state and is exposed to a flow of 100 standard liters per minute of hydrogen.
In general, in another aspect, the invention can include a fuel processor for a fuel system that includes a catalyst composite containing a first material capable of catalyzing an exothermic reaction and a second material capable of sorbing and desorbing water. The catalyst composite can be disposed within the fuel processor. In some embodiments, the fuel processor can be devoid of a high-temperature shift catalyst.
In general, in another aspect, the invention provides a low temperature water-gas shift reaction catalyst that can have one or more of the following features: it can be present in granules (such as pellets); it can include a copper based catalyst; it can include a zinc based catalyst or component; and it can include a desiccant material such as a zeolite in a weight percent equal to or greater than a combined first and second amounts of CuO and ZnO. As previously discussed, the granules can be present in multiple shapes and sizes. In general, a granule""s largest cross-sectional dimension is referred to as a cross-sectional dimension. For example, this would refer to the diameter of a spherical granule, or to the larger of the length and diameter of a cylindrical granule. In certain embodiments, the granules can have a cross-sectional dimension of about xe2x85x9 inch (this dimension can be larger as desired), and the desiccant can be present in an amount such that when about 20 kg of the granules are at a temperature less than 100xc2x0 C., and are exposed to a water-saturated flow of 100 standard liters per minute of hydrogen at a temperature less than 300xc2x0 C., the granules are limited to a temperature below about 400xc2x0 C. as the granules are subject to reduction from the hydrogen. In some embodiments the desiccant can be present in an amount such that when about 20 kg of the granules are saturated with water at a temperature less than 100xc2x0 C., and are exposed to a dry flow of 100 standard liters per minute of hydrogen at a temperature less than 300xc2x0 C., the granules are limited to a temperature below about 400xc2x0 C. as the granules are subject to reduction from the hydrogen. In some embodiments, the desiccant can contain water and be present in an amount such that the granules are limited to a temperature below about 400xc2x0 C. as the granules are exposed to air. In some embodiments, this exposure to air can include an air flow of about 4 liters per minute.
Many other embodiments are possible. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. All percents and ratios described are by weight.